This proposal constitutes a resubmission of a competitive renewal of a research program to design and apply octahedral metal complexes to target DNA with site-specificity. We are focused primarily on metalloinsertors, bulky metal complexes that bind to single base mismatches in DNA with high specificity through a unique binding mode. We have shown crystallographically that in this novel insertion mode, the metal complex binds DNA from the minor groove side, inserting its bulky ligand into the base stack and ejecting the mismatched bases into the major groove. We have also shown that these complexes display remarkable cell selectivity in inhibiting cellular proliferation preferentially in mismatch repair-deficient cells. Importantly, cells deficient in mismatch repair are associated with cancerous transformation. We therefore propose the application of these complexes that bind mismatches as the basis for a completely new strategy to develop diagnostics and chemotherapeutics targeted selectively to cells deficient in mismatch repair. No other family of small molecules thus far offers this opportunity. We will prepare new luminescent mismatch- targeting agents as early cancer diagnostics using luminescent Ru analogues for specificity coupled to organic fluorophores for brightness through resonance energy transfer. We will extend the family of mismatches targeted to include the more stable guanine mismatches by functionalizing the Rh complexes for hydrogen bonding into guanine-containing mismatches. We will also prepare a new Rh scaffold for bifunctional conjugates. Structural studies will be conducted for new complexes bound to their target sites. Experiments to monitor and optimize cellular uptake on luminescent ruthenium analogues will be conducted using flow cytometry, confocal microscopy and mass spectrometry. We will examine a family of Ru and Rh complexes, where the ancillary ligands are systematically varied with charged, lipophilic and polar functionalities and with appended peptides. Studies will include nuclear-localizing peptides that contain a cleavable linker once in the cell. Biological experiments, focused on comparisons between matched cell lines deficient versus proficient in repair, will be carried out to characterize cellular responses. We intend to optimize the cell-selective response using the complexes prepared by (i) increasing DNA affinity for the full family of mismatches; (ii) increasing nuclear uptake; and (iii) coupling the complexes with other reactive agents. Bifunctional conjugates will be made with an oxaliplatin derivative and with functionalities containing radionuclides to establish a potent cytotoxic response that is selective for cells deficient in mismatch repair. We will also probe the biological mechanism responsible for the cell-specific antiproliferative effect we observe in the absence (and presence) of conjugated cytotoxic agents.